Pressure-sensitive adhesives are used to bond optical films such as polarizing plates and retardation films to adherends such as liquid crystal panels, or to attach protection films to optical films. In the event of wrinkles, air trap, entrapment of foreign substances or misalignment, it is often necessary that the optical films or the protection films that are bonded are removed and re-bonded. Further, the optical films are removed to recover expensive liquid crystal panels for recycling. In view of these circumstances, pressure-sensitive adhesives for optical films that are used to bond optical films to adherends such as liquid crystal panels or to attach protection films to optical films, are required to be removed with appropriate peeling strength without adhesive transfer and are also required to have re-workability so that they can effect bonding even after removed.
Furthermore, it is necessary that pressure-sensitive adhesives for optical films have high durability such that they are not expanded or do not separate from adherends even when exposed to high temperatures or wet and hot conditions.
Various pressure-sensitive adhesives for optical films have been disclosed. (1) Patent Document 1 discloses pressure-sensitive adhesives for polarizing plates or retardation films that comprise a graft copolymer or a block copolymer which includes a high-Tg polymer segment having a glass transition temperature (Tg) of not less than 50° C. and a molecular weight of 500 to 1,000,000 and a low-Tg polymer segment having Tg lower than that of the high-Tg polymer segment by 70° C. or more wherein the molecular weight of the copolymer ranges from 400,000 to 2,000,000. (2) Patent Document 2 describes pressure-sensitive adhesives for optical films that contain 100 parts by weight of an acrylic polymer with a weight average molecular weight of not less than 1,000,000 and 1 to 40 parts by weight of an acrylic oligomer with a glass transition temperature of not more than −5° C. and a weight average molecular weight of 800 to 50,000. (3) Patent Document 3 discloses pressure-sensitive adhesives for optical films that comprise an acrylic-based polymer as a base material preferably having a weight average molecular weight of not less than 1,000,000 and an acrylic oligomer comprising a block copolymer which has a polymer block with a glass transition temperature thereof alone of not more than −5° C.
However, the working examples of Patent Document 1 only describe pressure-sensitive adhesives that are composed of a graft copolymer having a high-Tg polystyrene segment as a branch and a butyl acrylate polymer segment as a main chain. According to Patent Document 1, chemical crosslinking treatment is required in order that the pressure-sensitive adhesives show adhesive properties. For the crosslinking, functional groups such as hydroxyl groups and carboxyl groups are introduced in advance in the low-Tg polymer segment (backbone) constituting the graft copolymer that is the base material of the pressure-sensitive adhesive. The chemical crosslinking is achieved by adding a crosslinking agent (e.g., “Coronate L”, a trifunctional isocyanate compound) to a solution of the pressure-sensitive adhesive at the stage of the application of the pressure-sensitive adhesive. Accordingly, the chemical crosslinking of the pressure-sensitive adhesives disclosed in Patent Document 1 adds a post-treatment step in the production of pressure-sensitive adhesive optical films to lower productivity. Furthermore, bonding performance tends to vary due to uneven crosslinking.
In regard to the pressure-sensitive adhesives of Patent Documents 2 and 3, the acrylic polymers that are the base materials of the pressure-sensitive adhesives have a very high weight average molecular weight of 1,000,000 or more, resulting in high solution viscosity. To obtain pressure-sensitive adhesive solutions having low viscosity and high application properties, it is necessary that large amounts of organic solvents are used to lower the solid concentration in the pressure-sensitive adhesive solutions. The use of large amounts of organic solvents causes problems in terms of environmental pollution and extended time required to remove the solvents after application. Similar to the pressure-sensitive adhesives disclosed in Patent Document 1, Patent Documents 2 and 3 entail that functional groups such as carboxyl groups are introduced in advance in the acrylic polymers that are the base materials of the pressure-sensitive adhesive so that crosslinks may be formed and adhesive properties may be achieved by reaction with a crosslinking agent (e.g., “Coronate L”, a trifunctional isocyanate compound). The chemical crosslinking is achieved by adding a crosslinking agent to a solution of the pressure-sensitive adhesive at the application of the pressure-sensitive adhesive. Accordingly, the chemical crosslinking adds a post-treatment step in the production of pressure-sensitive adhesive optical films to lower productivity. Furthermore, bonding performance tends to vary due to uneven crosslinking.
Meanwhile, hot-melt pressure-sensitive adhesives containing acrylic triblock copolymers are known (Patent Documents 4 and 5). These documents, however, do not describe that the hot-melt pressure-sensitive adhesives are used for optical films nor that instead of being molten, the hot-melt pressure-sensitive adhesives are dissolved in organic solvents and the resultant pressure-sensitive adhesive solutions are used for optical films.    Patent Document 1: JP-A-H07-82542    Patent Document 2: JP-A-2003-329837    Patent Document 3: JP-A-2004-58289    Patent Document 4: JP-A-H11-302617    Patent Document 5: JP-A-H11-323072    Patent Document 6: JP-A-H06-93060    Patent Document 7: JP-B-H07-25859    Patent Document 8: JP-A-H11-335432    Non-Patent Document 1: Macromol. Chem. Phys., 2000, vol. 201, pp. 1108-1114